stand by

verb

1. intr, adverb to be available and ready to act if needed or called upon
2. intr, adverb to be present as an onlooker or without taking any action

   he stood by at the accident

3. intr, preposition to be faithful to

   to stand by one's principles

4. tr, adverb English law (of the Crown) to challenge (a juror) without needing to show cause

noun

1. 1. a person or thing that is ready for use or can be relied on in an emergency
   2. ( as modifier )

      stand-by provisions

2. on stand-by
   in a state of readiness for action or use

adjective

1. (of an airline passenger, fare, or seat) not booked in advance but awaiting or subject to availability